As is explained in document U.S. Pat. No. 5,095,706, a combined cycle power plant has been highly evaluated as a power generating system which assures that two excellent properties, i.e., a facility of load change and a high thermal efficiency can be achieved. To rise up a level of the excellent properties as mentioned above, research and development works have been conducted, so as to improve a method of operating a power plant and apparatuses and instruments associated with the foregoing method. The latest remarkable activity with respect to the method of operating a power plant is a shift from a base load operation to a daily start and stop operation (hereinafter referred to simply as a DSS operation).
During the shutdown of the steam turbine plant, a large quantity of condensate is stored in the hot well in the condenser in preparation of next start-up operation of the steam turbine plant. However, since atmospheric air intentionally by the vacuum breakers and unavoidably by leakages enters into the interior of the condenser during the shutdown of the steam turbine plant, a long period of waiting time elapses until operation of the steam turbine plant is started, as the condensate is brought in contact with the atmospheric air. For this reason, a large quantity of oxygen and CO2 becomes dissolved in the condensate with the result that the dissolved oxygen concentration varies to a very high value in excess of a preset value of 7 ppb, which should be maintained during operation of the steam turbine plant. For example, provided that no measure is taken not only to prevent an atmospheric air from penetrating the condenser but also to prevent oxygen in the atmospheric air from being dissolved in the condensate, a value of the dissolved oxygen concentration is raised up to a high value of 10000 ppb and the pH-value is increased.
Provided that the dissolved oxygen concentration which has been raised up to a level of 10000 ppb should be lowered to 80 ppb with the aid of the condensate recirculation system (numeral 160 in FIG. 7 of the U.S. Pat. No. 5,095,706), a long time is required to deaerate the condensate. Thus, there is a possibility that a time consumed for deaerating at every time of DSS operation is elongated and the steam turbine plant fails to quickly meet the requirement from the electricity consumers' side.
To solve this problem, the U.S. Pat. No. 5,095,706 provides a start-up method of a steam turbine plant including a condenser wherein a tube bundle for condensing an exhaust steam from a steam turbine and a hot well for storing a condensate therein are arranged in the interior of a condenser shell, wherein the interior of the condenser shell is air tightly divided into two parts with a partition plate there between, one of them being an upper space containing the tube bundle and the other one being a lower space containing the hot well; at least one connecting pipe extends between the upper space and the lower space, the connecting pipe being provided with an isolation valve to isolate the both spaced at the position there between; at the shut-down of steam turbine plant, the isolation valve on the connecting pipe is closed to isolate the lower space from the upper space and to maintain the lower space in vacuum and; at the start-up of the steam turbine plant, the isolation valve on the connecting pipe is opened after the upper chamber is evacuated, whereby the upper space is communicated with the lower space which has been maintained in vacuum.
Another way to establish the necessary vacuum in the condenser prior to opening of the steam turbine bypasses and prior to start up the steam turbine is done with evacuation pumps (normally steam ejectors or water ring pumps). Such a solution is for example disclosed in document WO 01/59265 A1.
However, these known operating methods are still time and energy consuming.